Where is radius parameter for sun lamp in newer version of blender 2.8?
The size of the sun is now "angle" 

The tooltip describes it as


  the angular diameter of the sun as seen from the earth. 


Making that value smaller will make the shadows sharper (a more defined umbra), and making it larger will result in softer shadows (or less penumbra).



Do a search online the concepts of Umbra and Penumbra.